A conventionally known gas sensor has a gas sensor element for measuring the concentration of a particular gas, such as NOx (nitrogen oxides) or oxygen, contained in exhaust gas from an automobile or the like on the basis of electromotive force which is generated according to the differential of concentration of the particular gas and whose magnitude varies with the differential of concentration. The gas sensor is mounted, for use, to an automotive exhaust pipe or the like. Since the gas sensor element becomes high in temperature through exposure to high-temperature exhaust gas and through application of heat from a heater or the like, for example, when water contained in exhaust gas, or condensed water adhering to the inner surface of the exhaust pipe adheres to the gas sensor element, the gas sensor element may crack or fracture from thermal shock.
Thus, there has been developed a technique for protecting the gas sensor element from adhesion of water by means of covering the gas sensor element with a protector (refer to, for example, Patent Document 1). This protector has a dual structure composed of an inner protector and an outer protector. A gas-to-be-measured is introduced into the protector through gas introduction holes formed in the inner and outer protectors. Also, gas discharge holes are formed in side walls of a recess portion of a front end wall of the inner protector, whereby a water droplet which comes flying from the ambient atmosphere (from the front side of the gas sensor element) is unlikely to come in direct contact with the gas sensor element. Also, according to this technique, a taper portion formed between a bottom wall and a side wall of the inner protector projects frontward from the outer protector.